In modern society, networks facilitate the rapid transference and exchange of a great amount of data, information, multimedia and knowledge in a digital and electronic form. This promotes interpersonal communication, accumulation of experience, knowledge exchange, and technological advancement. Thus, networks have already become a foundation of the modern information-age society. Wired networks already having had a broad, almost universal, foundation, wireless networks nowadays are also being developed rapidly, allowing users to access information sources at any time, in any place, in a mobile and portable way.
Please refer to FIG. 1, which is a functional block diagram of a computer system 10 of the prior art, capable of accessing wireless networks. The computer system 10 is designed with a central processing unit CPU0, a Northbridge chip NB0, a Southbridge chip SB0, memory 12, a graphics accelerator card 16, a display 18, a peripheral device P0, and a storage device M0. In order to access a wireless network 22, the computer 10 can be designed with a wireless network card 20. The central processing unit CPU0 is used to control operation of the computer 10; the Northbridge chip NB0 electrically connects the central processing unit CPU0, the memory 12, and the graphics accelerator card 16, and is used to manage a rapid information exchange between the three. The memory 12 is used to store, in a volatile fashion, information and programs needed while the central processing unit CPU0 is operating. The central processing unit CPU0 uses the graphics card 16 to process image data, and to send the image to the display 18, which then displays the image. The Southbridge chip SB0 connects the Northbridge chip to a plurality of buses 24 (such as PCI, IDE, or USB). The peripheral device P0 (such as a sound card), the non-volatile storage device M0 (such as a CDROM drive or a hard disk drive) and the wireless network card 20 are on the buses. The Southbridge chip SB0 primarily manages lower-speed information transfer between the central processing unit CPU0 and the devices connected to the buses.
The wireless network card 20 could be a network card that complies with the IEEE 802.11 wireless local area network (WLAN) specification. The wireless network card 20 is designed with a media access circuit MAC and a physical layer circuit PHY. Under the open system interconnection (OSI) architecture, the media access circuit MAC is used to implement media access control layers for the wireless networks. When the computer 10 is used to access network resources, the media access circuit MAC can use the corresponding physical layer circuit PHY1 to acquire digital information, process the information, and send the information to the computer. Information that the computer 10 sends to the network is packaged by the media access circuit MAC. The media access circuit MAC also arranges a physical location for the packaged information to access the network, and sends the packaged information to the physical layer circuit PHY. Similarly, the media access circuit MAC unpacks information received by the physical layer circuit PHY from the network.
The physical layer circuit PHY of the wireless network card 20 is used for providing wireless physical layer functionality. Information to be sent to the network, after being processed by the media access circuit MAC, is sent to the corresponding physical layer circuit PHY, which converts the information to a signal suitable for transmission, and transmits the signal. The physical layer circuit PHY can also receive signals from the network and unpack or demodulate the signals to acquire information contained in the signals, then send the information to the corresponding media access circuit MAC. The physical layer circuit PHY used for wireless network access further comprises a baseband circuit and a radio frequency (RF) circuit (not shown). The baseband circuit performs digital processing on information received from the media access circuit MAC, and then the RF circuit wirelessly transmits the information. RF wireless frequency signals received from the wireless network are received by the RF circuit. Then, the baseband circuit converts the demodulated signal into electronic information that is further sent to the media access circuit MAC.
When the computer 10 accesses network resources, all network cards work with a driver program stored in the memory 12. The driver program manages data transfer between the computer 10 and the network. A wireless network driver 26 is used with the wireless network card 20 to allocate a plurality of descriptors TxB (indicated individually as TxB(1) through TxB(n2)) and a plurality of descriptors RxB (indicated individually as RxB(1) through RxB(m2)) for pointing to a data transmission allocation DTB and a data reception allocation DRB. Information to be sent to the wireless network 22 is stored in the memory allocation DTB pointed to by the descriptors TxB, and information received from the wireless network 22 is stored in the memory allocation DRB pointed to by the descriptors RxB. For the computer 10 to access the wireless network, the media access circuit MAC and the central processing unit CPU0 use the descriptors TxB and RxB to access the information sent to the wireless network, and received from the wireless network, stored in the memory 12.
To accommodate special demands of the wireless network, the descriptors TxB and RxB used for accessing the wireless network must further indicate a particular status of the wireless network. For example, because the wireless network 22 and the computer 10 are not connected by a physical network cable, when the computer 10 transmits a large amount of wireless information to the wireless network 22, the computer 10 has no way of confirming that the information sent wirelessly by the wireless network card 20 has already been received smoothly by another computer on the wireless network 22. At this time, the central processing unit CPU0 requests that the wireless network 22 send to the computer 10 an acknowledgement of data having been received completely. In practice, when the central processing unit CPU0 uses the wireless network driver 26 to store the information to the memory allocation DTB, the central processing unit CPU0 arranges the descriptor TxB to point to the memory allocation DTB, and the descriptor TxB indicates the acknowledgement required by the wireless network 22. When the media access circuit MAC of the wireless network card 20 accesses the information according to the descriptor TxB, the media access circuit MAC uses the descriptor TxB to know to request acknowledgement from the wireless network 22. In this way, when the media access circuit MAC packages the information, the media access circuit MAC adds acknowledgement information to the head of the packet. The packet is then sent to the physical layer PHY, and the physical layer PHY sends the packet wirelessly to the wireless network 22.
Practically speaking, in an architecture such as IEEE 802.11, aside from the acknowledgement request, there are a number of settings that are different from those of the wired network. For example, as information is transmitted wirelessly, in addition to the transmitter and the receiver, any third party that can receive wireless signals could intercept the wirelessly transmitted information. In order to ensure that the information content does not leak, the IEEE 802.11 architecture provides a wired-equivalent privacy (WEP) mode, which encrypts transmitted and received information at both ends, and maintains basic information security. The central processing unit CPU0 also uses the descriptors TxB and RxB to govern whether or not the media access circuit MAC uses WEP to access network resources. Also, in order to adapt to a portable nature of computers employing the wireless network architecture, in the wireless network architecture, each computer connects to the network at an access point. More specifically, when the computer 10 accesses information on the wireless network 22, the computer 10 establishes contact to get a basic service set (BSS) organized by an access point. The basic service set can comprise a plurality of computers, all connected wirelessly to the BSS through connection to the access point. A physical wireless network address of the access station could act as a basic service set identification (BSSID). When a first computer in a first basic service set connects to a second computer of a second BSS, the first computer first contacts an access point of the first BSS. The access point of the first BSS connects to an access point of the second BSS through a distribution system service (DSS), and then connects to the second computer through the second access point, allowing the first computer to contact the second computer. In the wireless connection process just described, each computer must make contact with a respective access point, enter/exit the BSS, access the DSS through the access point, etc. And all of these matters occur between the computers and the access points with the help of management and control framework information, such as BSSID's, acknowledgements, and beacons.
For coordinating the work of the devices in the wireless transmission network as mentioned above, clock synchronization between the station and the access point is a quite important issue.